1. Field of the Invention
2. Description of the Prior Art
Emission of carbon dioxide (CO.sub.2) from electric power plants and other industrial boilers is a major contributor to climate changes and increases the global warming effect.
The vast majority of the carbon dioxide emissions into the atmosphere comes from burning fossil fuels which contributes to the problem of global warming. As the majority of the developing countries continue their fast rate of development, they will burn fossil fuels and emit carbon dioxide at much higher rates than ever. If no measures are taken to cap or slow down the emission of carbon dioxide; it will accelerate the climate warm up which is estimated to gain five to eight degrees Fahrenheit during the next century, and may cause catastrophic environmental effects due to the expansion of the ocean waters and melting of the polar ice cap. At the global warming summit of 1997 in Koyo, Japan, the U.S. and the International Community have started a global effort to protect the environment from the global warming effect. The U.S.A. is required by the year 2010 to reduce the carbon dioxide (CO.sub.2) emissions by a minimum of 15% down to the levels of 1990 emissions. The object is to achieve capping the greenhouse gas emissions without crippling the economy or increasing the cost of generating electricity. The goal has been established to stabilize the greenhouse emissions by year 2010 and has raised the concerns of the U.S. economists who claim that adopting a policy to slow the global warming effect will harm the American Standard of living; except if the measures would improve the productivity through increasing the utilization of energy efficient technologies and if new innovations are adopted to reduce the emissions of carbon dioxide into the atmosphere.
Known options for reducing CO.sub.2 emissions into the atmosphere include long term storage of CO.sub.2 gas underground or in the ocean, industrial utilization of CO.sub.2 emitted by the power plants, microalgae conversion of CO.sub.2 to biomass and foresting or tree planting.
Known concepts to achieve the reduction of carbon dioxide emissions from electric generating power plants are substituting natural gas for coal because burning coal produces much more carbon dioxide than natural gas; gasification of coal; increasing the use of nuclear power; increasing the use of renewable energy and conserving the use of energy.
Coal is a major natural resource in the U.S., Eastern Europe, China, and many other countries around the globe. In the U.S. 55% to 60% of the electric power generated is from coal fired power plants. Coal gasification processes and equipment are commercially available for the production of synthesis gas which bums cleaner and emits less carbon dioxide; however, they are based on using oxygen from air for the gasification reactions which make them very expensive and can not compete with the less expensive natural gas fuel reserves.
In my U.S. Patents; I disclosed methods and systems for developing a high efficiency/low emissions coal fueled power plant boiler system with improvements to the boiler flue gas cleaning placed down stream from the flue gas particulate removal equipment, the improvements which are referred to as "flue gas cleaning by condensing and liquefaction of sulfur dioxide and carbon dioxide". The present invention provides another step and discloses a process in which carbon dioxide extracted from the flue gas is recycled and utilized for gasification of the coal fuel to produce synthesis gas and char. The synthesis gas and char may be burned in a conventional boiler to produce steam for a steam turbine-generator to generate electric power while the synthesis gas may be burned in a gas turbine to generate electricity in combination with a steam turbine/generator.
Since the 1950's oxygen and steam are used for coal or biomass solid fuel gasification. The coal gasification chemistry reactions are well known and are generally demonstrated as follows: EQU C(coal)+O.sub.2 (air).fwdarw.CO.sub.2 +heat 1 EQU C(coal)+H.sub.2 O(steam).fwdarw.CO+H.sub.2 2 EQU C(coal)+CO.sub.2 .fwdarw.2CO 3 EQU CO+H.sub.2 O(steam).fwdarw.CO.sub.2 +H.sub.2 4
Reaction 3 indicates an intermediate step in the oxygen based gasification process where one molecule of carbon dioxide extracted from the flue gas reacts with one molecule of carbon in the coal to produce two molecules of carbon monoxide. Carbon monoxide is the main ingredient for producing synthesis gas. In oxygen or air based combustion gasifiers, oxygen in the flue gas slow down or inhibit the reaction to produce carbon monoxide in the gasification process, effectively this prevents de-association of the carbon dioxide into carbon monoxide and decreases the efficiency of the gasification process. Psychometric control of oxygen inside the oxygen based gasifier is very critical to maximize formation of carbon monoxide and hydrogen.
The oxygen based coal gasification process has been demonstrated in several power plants employing the well known integrated gasification combined cycle (IGCC) where low BTU synthesis fuel is produced and burned directly in a gas turbine for generating electric power. Residual heat in the exhaust of the gas turbine is used to generate steam for conventional steam turbine generators to produce more electricity in combination with power generated from the gas turbine system.
At the Tampa Electric Company in Polk County Florida, an Integrated Gasification Combined Cycle (IGCC) where coal water slurry and oxygen are combined at thigh temperature and pressure to produce a high temperature synthesis gas directly fired in the gasifier to produce hot flue gas and steam.
At the Wabash River Coal Gasification project, Terre-Haute, Indiana, the coal is ground, slurried with water and gassified in a pressurized two stage oxygen blown gasifier. The hot fuel gas is cooled in a steam generator heat exchanger to produce high pressure steam utilized to drive a steam turbine driving an electric generator. The cooled fuel gas is then fired in a gas turbine which drives another electric generator.
At the Pinon Pine IGCC power project of the Sierra Pacific Power Company an air blown, pressurized, fluidized gasifier technology was used to produce low BTU gas at 295 psia and 1800.degree. F. In conventional air blown/combustion fluidized bed gasifiers the synthesis gas product is diluted with nitrogen or with the combustion of flue gases. Oxygen blown gasifiers are typically used to reduce nitrogen and produce higher heating value of product gas, however the cost of oxygen separation plants is prohibitive.
In all these and other prior art technologies; oxygen is used for the direct oxidation of carbon to form carbon monoxide at a high temperature range in a pre-combustion stage. This reaction could be inhibited by the presence of excess oxygen in the gasifier which limits the gasification process and yields char with a large percent of the solid carbon not gasified but remains un-reacted.
In the present invention, the gasification of coal is achieved by using carbon dioxide and steam in the absence of oxygen. At elevated temperatures and pressure, preferably in the range of 1400 to 1800.degree. F and up to 350 psig, carbon dioxide dis-associates and radical oxygen reacts with the carbon in the char to form carbon monoxide. The separation of the gasification reactions and the combustion into two steps achieve much higher efficiency and lower emission levels along with higher Btu synthesis gas products.
In the absence of oxygen, carbon dioxide defuses through the char layer gasifying the char with gasification reaction as follows: EQU C+CO.sub.2 .fwdarw.2CO
The reaction is known to be an endothermic reaction which requires heat input supplied by the partial oxidation of carbon from the immediate vicinity of the char solid particle.
In a similar reaction and under the same circumstance H.sub.2 O (steam) reacts with the carbon as follows: EQU C+H.sub.2 O.fwdarw.CO+H.sub.2
The thermo-chemical reactions of carbon dioxide and steam with carbon yield carbon monoxide and hydrogen without the need of oxygen for gasification. Both carbon monoxide and hydrogen are combustible gases and are the basic ingredients to form synthesis gas which is known as water gas of a general formula (CO--H.sub.2). Part of the synthesis gas can be used to provide supplemental heating in the gasifier using indirect fire tube furnace or pulse heating tube banks projected inside the bulk of the gasifier.
The present invention takes advantage of the reaction of carbon dioxide with the carbon in the coal to reduce the carbon emissions and increase the fuel utilization in coal fired or biomass fueled power plants. This innovative technology will support the need to continue using the coal as a viable domestic fuel. It will be economically feasible to repower conventional pulverized coal power plants for both atmospheric and pressurized steam boiler systems with synthesis gas and char produced from carbon dioxide gasification of coal in an Integrated Gasification Combined Cycle (IGCC) configuration to achieve the goal of high efficiency/low emission coal fired power plants.
In so far as is known the present invention as described herein is the first to employ carbon dioxide extracted from the boiler flue gas for the gasification of coal or biomass solid fuels.